In the field of optical discs such as CDs and DVDs, CD-Rs, DVD-Rs to which data can be recorded only once, and CD-RWs, DVD-RWs, DVD-RAMs, and the like, which can be rewritten, are widely spread as the recording type optical disc media, in addition to the reproduction-only ROM media on which embossed data pit arrays are formed.
The recording type optical disc medium is created by forming a recording track constituted with a spiral pre-groove used for tracking on an optical disc substrate, and forming a multilayered recording layer made of an organic material or the like thereon. Through collecting high power laser beams on the recording layer, the recording layer is partially changed in quality to form a recording pit for recording the data thereby. With the CD-R and DVD-R, it is possible to obtain servo signals with the almost same characteristic with the same data format structure as that of the embossed ROM medium after recording. Thus, it can be easily reproduced by a reproduction-only drive device.
For example, in the case of a DVD-R, it is possible to detect a track error by a push-pull signal by using pre-groove under a non-recorded state, and to detect the track error by retardation detection signal by using the pit as in the ROM after the data is recorded.
Incidentally, in order to achieve fine recording and reproduction with an optical disc device, it is necessary to pre-record so-called control data information that indicates the type and characteristic of the optical disc medium on the recording type optical disc medium (for example, Patent Literature 1). Hereinafter, the recording type optical disc medium is simply referred to as an “optical disc medium”.
As shown in FIG. 5, in a conventional optical disc medium 50, normally, an area called a lead-in area 53 is provided on the inner peripheral side of a data recording area 52 where a recording track 51 is provided. Before shipping out the optical disc 50, the control data is recorded in the lead-in area 53. The optical disc device reproduces the control data first, and performs tracking operation in the data recording area 52 thereafter.
In a DVD-R medium, for example, written as the control data information is only the simple information indicating at what speed the data can be recorded to the optical disc medium. The optical disc device can read this information, and perform the recording operation at a corresponding speed.
Patent Literature 1: Japanese Unexamined Patent Publication 2004-327013
Incidentally, in the DVD-R, the CD-R, and the like, which are widely used in general, there is set an allowable characteristic range of the optical disc medium so that the optical disc device can achieve a stable operation for the optical disc media of any manufacturers. This setting range is organized in a standard book, which serves as a standard specification common to the manufacturers of optical disc devices and manufacturers of optical disc media. For DVD-R (3.95 GB), for example, it is issued as ECMA-279 standard.
One of those set in the characteristic range as described above is a push-pull signal that is a track error detected from the pre-groove. FIG. 6 illustrates the principle for detecting the track error by the push-pull signal. As shown in FIG. 6B, when a convergent beam spot 61b is irradiated from an optical head to the center of a pre-groove 62 of the recording track, reflected light distribution 63b detected by the optical head becomes laterally symmetrical. In the meantime, as shown in FIGS. 6A and 6C, when convergent beam spots 61a, 61c are deflected on the left side or the right side with respect to the pre-groove 62, reflected light distributions 63a, 63c become laterally asymmetrical accordingly.
When those reflected light distributions 63a-63c are received by a bipartite photodetector that has a split line in the center, a difference signal of I1−I2 generated based on the two output signal I1, I2 becomes a push-pull signal. The push-pull signal becomes a track error detection signal, which indicates the position shift between the converged beam spots 61a-61c and the pre-groove 62.
FIG. 7 is a waveform chart indicating a change in the output signal of the bipartite photodetector, when the converged beam spot is shifted in a direction traversing a plurality of recording tracks. The difference signal is almost a sin wave detection signal that becomes almost zero at the center position of the track. Meanwhile, the sum signal I1+I2 obtained by adding the two output signals becomes a detection signal with less change.
When a peak value of the amplitude of the difference signal (I1−I2) is divided by a mean value of the sum signal (I1+I2) obtained at the time of traversing the track, which is expressed as follows, there is obtained a signal called a standardized push-pull signal.(I1−I2)p−p/(I1+I2)DC  (1)The range of numerical values the standardized push-pull signal can take is set by considering the operation margin of the optical disc device.
FIG. 8 is a block diagram for showing the part in the optical disc device, which performs tracking operation by the push-pull signal. In the followings, explanations will be provided by referring to FIG. 5 and FIG. 8.
Emitted beams from the laser light source 71 form a converged beam spot on the recording track 51 of the optical disc medium 50 through a lens with drive mechanism 72. The reflected light from the converged beam spot is isolated at a mirror 73 and received by a bipartite photodetector 74. Two outputs signals of the bipartite photodetector 74 generate a push-pull signal at a differential circuit 75, and this push-pull signal is sent to a servo drive circuit 76. The servo drive circuit 76 drives the lens with drive mechanism 72 in a direction orthogonal to the track in accordance with the shift amount of the detected convergent beam spot position thereby to correct the shift between the convergent beam spot and the recording track. By such an operation called a feedback servo control, tracking operation of the recording track 51 is carried out.
The important thing in such servo control is to optimize the gain of the servo drive circuit 76. If the gain is too large, the gain as the feedback loop may become too high, which may cause oscillation. Meanwhile, if the gain is too small, the gain as the feedback loop becomes insufficient, which increases the possibility of causing the tracking to be shifted off.
When the standardized push-pull signal changes, the gain as the feedback loop becomes changed. Thus, it is necessary to set the allowable range for the standardized push-pull signal, so that there is no problem generated in the operation of the optical disc device. Normally, it is so set that a change of about ±3 dB for the center gain can be permitted. Accordingly, the allowable range for the standardized push-pull signal is set in such a manner that the maximum value becomes about the twice the minimum value. For example, in the case of DVD-R under the state with no recording pit, the minimum value for the standardized push-pull signal is set as 0.22 and the maximum value as 0.44.
In the conventional CD and DVD, it is sufficient to set a single range for the push-pull signal. However, in an optical disc medium such as the one using blue laser with the wavelength of 400 nm band, for example, various recording materials such as organic types or inorganic types are to be used depending on its usage. Thus, it has become difficult to suit all the optical disc media within the double allowable range. It is possible to widen the allowable range to be more than the double value. With that, however, it becomes difficult to secure the stability of the optical disc device because of the characteristic of the servo control as described earlier.
Furthermore, there is also an issue in the conventional method for recording the control data. There is a method which also forms a recording track with the same pre-grooves as those of the data recording region, and records control data information at the manufacturer of the optical disc media before shipping the products. However, this method is costly since it requires time and work for recording it on each piece.